The present invention relates to a turbomolecular pump, and more particularly to rotor blades of a "turbomolecular" pump.
In a conventional turbomolecular pump, such as shown in FIG. 5, a plurality of stator blades 2 are axially disposed on the inner wall surface of a substantially cylindrically shaped casing 1. A rotor 3 is mounted inside the stator blades. A plurality of rotor blades 4 are arranged regularly alternately with the stator blades 2 on the outer wall surface of the rotor 3.
The rotor 3 is held by magnetic bearing means comprised of an axial electromagnet 6 and a radial electromagnet 7 provided on a hollow stator column 5. The rotor 3 is held floated radially and axially by the magnetic bearing means.
The stator column 5 is further equipped with a radio frequency motor 8 to rotate the rotor 3. The axial position and the radial position of the rotor 3 are detected by sensors 9 and 10, respectively. Protective dry bearings 11 and 12 are mounted over and under respectively, the stator column 5 to prevent the magnetic bearing from colliding against the rotor 3 when the magnetic bearing is suddenly de-energized due to power failure or malfunctions of the control circuit.
The rotor 3 is rotated at a high speed to induce streams of gaseous molecules between the successive stator blades 2 and rotor blades 4 to obtain an ultra high vacuum.
In a turbomolecular pump of the type described above, the rotor 3 has slotted rotor discs to form rotor blades 4 as shown in FIG. 6 and FIG. 7. The rotor blades are inclined relative to the plane of the rotor 3 with an optimum blade angle .alpha. as shown in FIG. 8 which is constant from the base to the outermost end of the rotor blade 4. The pumping speed is determined by parameters such as opening ratio .epsilon., relative blade interval .lambda., and the relative speed of gaseous molecules with respect to the revolution speed of the rotor blades, wherein, referring to FIG. 4, the opening ratio .epsilon. is defined by S1/(S1+G), and the relative blade interval .lambda. is defined by S2/b.
In a conventional turbomolecular pump, only one rotor blade angle which is optimum at one point along the radial length of a rotor blade is selected. Rotor blades are then formed with this constant rotor blade angle. Since the blade angle is constant, the above parameters change along the rotor blade depending upon the distance from the center of the rotor 3. Even though the blade angle and the parameters are optimum at one point along the rotor blade, they are not optimum at other points, e.g. near the base or the outermost end of the blade. Therefore, the uniform blade angle does not produce an optimum pumping speed along the whole length of the rotor blades.